1. Field of the Invention
This invention relates in general to improved rotary-percussion earth boring bits and in particular to improvements in the geometric relationship of the bit and the air course arrangement to minimize fatigue failures and to reduce costs of manufacture.
2. Description of the Prior Art
The first known commercially successful rotary-percussion earth boring bit of the type having button inserts is described in U.S. Pat. No. 3,185,228, which issued to Joseph L. Kelly, Jr. on May 25, 1965. While such bits have been commercially successful, fatigue failures often occur before the button inserts serve their useful life. Fatigue failures may occur in a mid-region of the body of the bit, where cross-sectional areas vary to provide torque transmission means such as splines used to rotate the bit. Other fatigue failures occur in a lower region of the bit body, commonly near the intersection of passages through which air flows to cleanse cuttings from the bottom of the borehole.
A number of solutions to the above fatigue failure problem have been suggested in the prior art. Large radii have been used at surface intersections to minimize stress concentrations. Air courses have been reamed or ground to minimize stress raising machine marks or other imperfections. The number of air courses has been decreased. As shown in U.S. Pat. No. 3,791,462, which issued to Alfred R. Curington on Feb. 12, 1974, a single air course of two drilled holes which intersect near the mid-region of the bit has been proposed. A similar air course construction is shown in the U.S. Pat. No. 2,579,268, which issued to Johannes A. S. Malherbe on Dec. 18, 1951. Previously, one central air course, coaxial with the axis of rotation of the bit, was utilized but discontinued due to its inability to remove the earth at the center of the borehole. As a consequence, fluid flow was restricted through the air course and the effectiveness of the air operated hammer, which drove the bit, was diminished.